[gvarndell]

I get the impression this young lady's role as presenter was a foregone conclusion before it was announced.
There could be very good reasons why she was a better choice, for that specific role, than you.
Many teens are just not good at interacting with adults; maybe she is and the mentor felt you would not be.
I think the mentor could/should have anticipated the emotional fallout and exercised better people skills.
You should not be wondering why you were not chosen.
Even if explaining the reason would have been awkward for the mentor.
Even if the reason doesn't seem at all fair to you.
As others have suggested, talk to other mentors about your feelings in this matter -- but be prepared to listen as well.
This may have been the right decision handled the wrong way.